1. Field of the Invention
This invention resides in the field of anisotropic thermal barriers and heat-spreading materials and to materials with anisotropic electrical conductivity, and further relates to ceramics, particularly of nanocrystalline structure, and carbon nanotubes.
2. Description of the Prior Art
Anisotropic thermal diffusivity is a quality that is sought in materials that are used in a wide range of applications, extending from a miniature scale such as components in the microelectronics industry to those of a relatively large scale such as structural components in aircraft engines. Coatings, layers, or components that conduct heat at a lower rate in one direction than in another direction that is orthogonal to the first are useful in dissipating heat to avoid localized hot spots or to insulate the surfaces of components from high-temperature environments. Heat dissipation, or heat-spreading, layers or coatings are used in microprocessors and integrated circuit packages to overcome the increased risk of damage due to the high power of the circuitry elements. The heat sinks in these devices must be lightweight and compact and yet able to absorb the heat from exothermic (heat-generating) electronic components in a highly efficient manner. The interposition of a heat-spreading layer between the exothermic element and the heat sink will cause the heat to travel laterally to extend over a broad heat sink area, adding significantly to the effectiveness of the heat sink. On a larger scale, the ability to spread heat laterally is useful in thermal barrier coatings such as those commonly used on high-temperature components in turbine engines as well as high-temperature reactors and spacecraft.
Thermal barrier coatings of anisotropic thermal conductivity have been formed by various methods, examples of which are thermal spraying, electron beam-physical vapor deposition, plasma spraying, and the use of materials such as graphites that possess a natural anisotropic crystal orientation that can be enhanced by roll pressing. Other anisotropic thermal materials include sintered aluminum nitride, carbon/carbon fiber composites, aerogels impregnated with ceramic fibers such as zirconium dioxide, silicon nitride, silicon carbide, alumina, and zirconium aluminate, and multilayered materials of various kinds.
Anisotropic electrical conductivity is likewise sought in many applications. These include electronic devices where static is sought to be eliminated by dissipating electric charges in a plane without transmitting them across the plane, ultra-high density circuit interconnections in which the close proximity of the leads raises a risk of short circuits, solder-free connections, thermoelectric applications where a maximal rate of heat transfer in a particular direction is sought by restricting the flow of electricity to one axis, photovoltaic cells where high lateral electrical conductivity is needed to transmit an electrical current to the edges of the cells without loss in the transverse direction, and high-temperature superconductors. Methods of achieving the desired anisotropy have included the use of anisotropic conductive adhesives, as disclosed by Jiang, T., et al., U.S. Pat. No. 6,579,744 (Jun. 17, 2003), the use of discontinuous conductors, as disclosed by Paul, W., et al., U.S. Pat. No. 6,475,958 (Nov. 5, 2002), polymeric foams, as disclosed by Vaccaro, A. J., et al., U.S. Pat. No. 5,738,907 (Apr. 14, 1998), specialized metallization patterns, as disclosed by Burgers, A. R. et al., U.S. Pat. No. 6,573,445 (Jun. 3, 2003), and layered crystal structures, as disclosed by Koumoto, K., et al., “Thermoelectric properties of single crystal CuAlO2 with a layered structure,” J. Mater. Chem. 11: 251-252 (2001).